Broadband amplifiers are key components in fiber-optic communication systems, future millimeterwave communication systems, and broadband test instruments, to name a few. Distributed amplifiers (DAs) have dominated broadband amplifiers design for the last two decades and have proved sufficient to realize broadband signal amplification covering multi-octave frequency bands. For instance, a Traveling Wave Matching (TWM) design technique has been developed from the DA concept for broadband matching and has been applied to the design and demonstration of several single stage DAs in K. L Koon, et al., “High Gain and Ultra Wideband SiGe/BiCMOS Cascaded Single Stage Distributed Amplifier for 4G RF Front-End Applications,” 14th IEEE 2003 International Symposium on Personal, Indoor and Mobile Radio Communication Proceedings, pp. 2180-2184 and in Virdee, A. S, et al., “Experimental Perfdrmance of Ultrabroadband Amplifier Design Concept Employing Cascaded Reactively Terminated Single-stage Distributed Amplifier Configuration,” Electronics Letters, Volume 36, Issue 18, Aug. 31, 2000, pp. 1554-56. Previously, a series of resistors, inductors and capacitors (R, L and C) was employed in each stage of the cascode distributed amplifier implemented with InP HEMTs in H. Shigematsu et al., “A 49-GHz preamplifier with a transimpedance gain of 52 dBΩ using InP HEMTs,” IEEE J. Solid State Circuits., vol. 36, no. 9, pp. 1309-1313, September 2001. A high gain-bandwidth product was reported, however, these amplifiers consumed large power and were larger in size due to the distributed amplification technique. In most DA amplifiers, the operable bandwidth that can be achieved is typically between one-tenth and one-third of the cutoff frequency (fT) of the amplifier, i.e., the frequency at which the transistor current gain in the amplifier is equal to one. Thus, to achieve a high bandwidth, the amplifier typically requires a high fT. However, fabrication costs for high fT amplifiers are great due to poor yields and other fabrication difficulties.
Many publications have reported broadband amplifiers with high gain-bandwidth products. To achieve high gain-bandwidth products in broadband amplifiers, one typically relies on either device technology, i.e. using transistors with very high fT values and maximum frequencies (fmax), circuit topology, i.e. designing distributed amplifiers (DAs), or operating transistors with high current density to achieve high fT and fmax. These options typically lead to more expensive technology, higher dc power consumption, and poor reliability, respectively.
Therefore, a broadband amplifier capable of achieving a high gain-bandwidth product while using typical transistors operated at typical current densities for biasing is needed, which will alleviate the reliability, dc power consumption and cost concerns of the conventional implementations.